Semiconductors form the basis of modern electronics. Possessing physical properties that can be selectively modified and controlled between conduction and insulation, semiconductors are essential in most modern electrical devices (e.g., computers, cellular phones, photovoltaic cells, etc.). Group IV semiconductors generally refer to those elements in the fourth column of the periodic table (e.g., carbon, silicon, germanium, etc.).
In general, a solid semiconductor tends to exist in three forms: crystalline, polycrystalline, and amorphous. In crystalline form, semiconductor atoms are positioned in a single unbroken crystal lattice with no grain boundaries. In polycrystalline form, the semiconductor atoms are positioned in many smaller and randomly oriented crystallites (smaller crystals). The crystallites are often referred to as grains. In amorphous form, the semiconductor atoms show no long-range positional order.
In general, conduction generally refers to the movement of electrically charged carriers, such as electrons or holes (i.e., lack of electrons), through a transmission medium. Metals tend to have substantial amounts of electrically charged particles available, whereas insulators have very few.
In the absence of impurities (called dopants), a semiconductor tends to behave as insulator, inhibiting the flow of an electric current. However, after the addition of relatively small amounts of dopants, the electrical characteristics of a semiconductor can dramatically change to a conductor increasing the amount of electrically charged carriers.
Depending on the kind of impurity, a doped region of a semiconductor can have more electrons (n-type) or more holes (p-type). For example, in a common configuration, a p-type region is placed next to an n-type region in order to create a (p-n) junction with an electric field. Consequently, electrons on the p-type side of the junction within the electric field may then be attracted to the n-type region and repelled from the p-type region, whereas holes within the electric field on the n-type side of the junction may then be attracted to the p-type region and repelled from the n-type region. Generally, the n-type region and/or the p-type region can each respectively be comprised of varying levels of relative dopant concentration, often shown as n−, n+, n++, p−, p+, p++, etc.
In another example, a junction may be created by placing an intrinsic (undoped) intrinsic semiconductor layer between the n-type region and the p-type region in order to mitigate the effects of quantum tunneling, a quantum-mechanical effect in which an electron transitions through a classically-forbidden energy state. For example, without an intrinsic separation layer, if the p-n junction is small enough, an electron can travel against the electric field and degrade the performance of the p-n junction.
In yet another example, a metal junction may be created by placing the n-type region and/or the p-type region next to a metal region in order to form an ohmic (low-impedance) contact.
One method of adding the impurities into the semiconductor involves depositing a doped glass on a semiconductor substrate, such as a Si wafer. Once exposed to relatively high temperature (e.g., 900-1000° C.), the dopants will tend to diffuse from the highly-doped glass into the substrate.
In addition, the high temperature also tends to anneal the substrate. Annealing is generally the process of heating a material above a critical or recrystallisation temperature in order to reduce the materials internal stresses, and or improve its physical and electrical properties. In the case of a semiconductor substrate, annealing allows the dopant atoms to properly position themselves in the lattice, such that the additional electrons or holes are available for the transmission of current. This is generally called activation and is critical for the creation of an efficient junction.
However, depositing doped glass may be problematic. For example, doped glass is often applied via a silk-screen. Silk-screening is generally a printing technique that makes use of a squeegee to mechanically force a liquid, such as a highly doped glass paste, directly onto a substrate. Consequently, this downward mechanical force tends to subject the substrate to additional stresses, and hence may detrimentally affect the electrical and physically characteristics of the substrate. In addition, creating alternating n-type and p-type regions on the same side of the substrate with a doped glass, such as with back contact solar cells, is difficult without the use of multiple and costly time-consuming printing steps.
Another method of adding the impurities into the semiconductor involves depositing dopants in a crystalline or polycrystalline substrate through ion implantation. Ion implantation generally accelerates dopant ions into the substrate at high energy. Like diffusion doping, the substrate must also generally be annealed at a high temperature to repair the substrate and activate the dopants. However, although dopant dosage may be controlled with high precision, ion implantation tends to be very expense since it requires the use of specialized and expensive semiconductor manufacturing equipment.
In yet a third technique of adding the impurities into the semiconductor, doped (thin) film layers may be formed using deposition techniques such as chemical vapor deposition (CVD). In a typical CVD process, a substrate (which can be an insulator, a semiconductor, or metal) is exposed to one or more volatile precursors, which react and/or decompose on the substrate surface to produce a doped film. However, like ion implantation, CVD is expensive since it requires specialized and expensive semiconductor manufacturing equipment. In addition, CVD also tends to be very slow, as the film layers are built up a single atom at a time.
In view of the foregoing, there is desired a method of producing low cost and efficient junctions for electrical devices, such as solar cells.